add kernel cross build support for faster builds on x86_64-linux

[DavHau]

This adds the option raspberry-pi-nix.kernel-build-system, which can be used to drastically decrease the build times.

Description of the option:

The build system to compile the kernel on.

Only the linux kernel will be cross compiled, while most of the derivations are still pulled from cache.nixos.org.

Use this in any of these cases:

• you are building on an x86_64 system using binfmt_misc for aarch64-linux.
• or if your x86_64 builder has a better CPU than your aarch64 builder.

[DavHau]

Damn, there still seems to be an error building modules-shrunk:

       > root module: tpm-crb
       > modprobe: FATAL: Module tpm-crb not found in directory /nix/store/v3r107514clbb7wnx3w9i7bkyqf519x5-linux-rpi-aarch64-unknown-linux-gnu-6.6.54-modules/lib/modules/6.6.54
       For full logs, run 'nix log /nix/store/31yh7hrkmvs0xx2vmpvs190nrkiysig0-linux-rpi-aarch64-unknown-linux-gnu-6.6.54-modules-shrunk.drv'.

[DavHau]

Damn, there still seems to be an error building modules-shrunk:

       > root module: tpm-crb
       > modprobe: FATAL: Module tpm-crb not found in directory /nix/store/v3r107514clbb7wnx3w9i7bkyqf519x5-linux-rpi-aarch64-unknown-linux-gnu-6.6.54-modules/lib/modules/6.6.54
       For full logs, run 'nix log /nix/store/31yh7hrkmvs0xx2vmpvs190nrkiysig0-linux-rpi-aarch64-unknown-linux-gnu-6.6.54-modules-shrunk.drv'.

Actually this problem was unrelated, and due to a mix of different nixpkgs in my config.

[DavHau]

I think this is good to go

[lluchs]

This worked for me, thanks!

[lluchs]

I think this is supposed to say rpi-kernels-cross

[ElvishJerricco]

In my configurations, I use something like this:

final: prev:
let
  inherit (final) lib;
in
{
  pkgsUncross = lib.flip lib.mapAttrs final.lib.systems.examples (
    _: localSystem:
    import final.path {
      inherit (final) config overlays;
      inherit localSystem;
      crossSystem = final.hostPlatform;
    }
  );
}

Which allows me to just set boot.kernelPackages = pkgs.pkgsUncross.gnu64.someKernelPackages;. First of all, I think this is significantly cleaner than having an ad-hoc nixpkgs import as you do in this PR currently, because it allows the use of pkgsUncross to cross compile anything as the user sees fit. Secondly, you'll notice I provided a much fuller set of arguments in the import than you do; preserving the config, overlays, and hostPlatform makes the package set much more accurate to what the user might be expecting. For instance, if I set the linux-kernel.target in my hostPlatform, that wouldn't work without passing along the original hostPlatform, which is important if you're using something like EDK2 or u-boot's EFI implementation to get compressed kernels with EFI_ZBOOT.

  nixpkgs.hostPlatform = lib.recursiveUpdate (lib.systems.elaborate lib.systems.examples.aarch64-multiplatform) {
    linux-kernel.target = "vmlinuz.efi";
    linux-kernel.installTarget = "zinstall";
  };

But I'm sure there's other, less obscure edge cases.

I've thought about upstreaming pkgsUncross into nixpkgs, but I don't think that's the name I would want to use if I were to upstream it. I just liked the name because it's sorta the reverse of pkgsCross.